Modern digital data systems typically represent information as variable patterns of two (or more) state cells called "bits". The bits are usually arranged in fixed-length patterns called data "words". Each word corresponds to the particular piece of data which is to be transmitted, recorded, or processed by digital apparatus. In order to transmit information from one point to another or record information on a storage medium, a serial stream of data bits representing an information flow through a transmission channel or into a storage medium is processed by the apparatus. However, in order to transmit or store a large quantity of information in a short time the number of bits per second processed by the transmission channel or the storage device must be very high. Frequently it is desired to transmit or record information at a higher rate than the capacity of present day transmission channels or recording systems. One way to transmit or record information at a high rate without exceeding the device information processing capability is to divide the data stream which is to be transmitted or recorded into a number of "channels" or "tracks" which are then transmitted or recorded in parallel This division results in a transmission or recording rate on each channel that is lower than the channel capacity yet the overall transmission or recording rate is acceptably high.
As an example, a high rate data stream may be recorded on multiple tracks of a magnetic tape. The incoming data stream is divided into several parallel data streams by a well-known multiplexing arrangement so that a first data word is recorded on a first track, the next data word is recorded on a second track, and so on. The tracks are all located on the same tape and are therefore maintained in the proper sequence. When the data is to be read off the tape the first word on the first track is read off, the corresponding word on the next track is read off next in sequence until the data stream is reconstructed. However, in a multiple track recording arrangement several problems arise. One serious problem is "skew". A skew condition results when data words which are supposed to be recorded in corresponding positions in each track start at different points on the tape. When a skew condition is present the circuitry which reads the information from the magnetic tape must be able to decide which words correspond so that the words can be read off in the proper sequence. Other problems often occur, for example, there are often random errors on the tape caused by malfunctions of the tape recorder circuitry or noise. These errors must be detected and corrected. Another problem is "track dropout" which occurs when a track stops recording for some reason, for example, circuit failure or imperfections in the magnetic tape. The data tracks must also be synchronized from track to track. Finally, the nature of the data itself may result in failure of the reproduction system. For example, a long string of logical "0"s or "1"s may result in an effective D.C. level in the data which cannot be processed by the recording or transmitting circuitry. Thus the encoding circuitry must be able to control the frequency spectrum of the data.
Therefore, when digital data is recorded on multiple tracks of magnetic tape, the following problems must be solved for a faithful reproduction of the recorded data:
1--Track-to-track skew must be detected and corrected PA1 2--There must be initial synchronization of all multiple tracks PA1 3--There must be detection and correction of a loss of synchronization on each track PA1 4--There must be detection and correction of isolated (random) errors and burst errors on multiple tracks PA1 5--There must be frequency spectrum control.